Casco Bay Lines plans to replace the Machigonne II, which is about 31 years old. A $6 million federal grant will help it do that.

Casco Bay Lines will get a $6 million federal grant to replace one of its aging passenger ferries.

Members of Maine’s congressional delegation announced the Federal Transit Administration award to the Portland-area public ferry service on Thursday.

“This funding is crucial to ensuring the ferry system will be able to continue safe and reliable operations in Casco Bay for many years to come,” said Sen. Susan Collins, R-Maine, in a statement. The ferry grant was supported by Collins, Sen. Angus King and Rep. Chellie Pingree, D-1st District.

The money will pay for more than half the $10 million ferry replacement, with other government grants and a local match paying the rest, said Casco Bay Lines General Manager Hank Berg.

The ferry service intends to replace the Machigonne II car ferry and Maqouit II passenger ferry in the near future, Berg said.

The Machigonne is about 31 years old and the Maquoit is 24 years old. The service has five vessels, the newest was built in 2013.

“We have been planning for the need to replace two of our ferries for some time now,” Berg said. “Both are about 30 years old and will reach the end of their useful life in about five years.”

The ferry service’s board of directors will vote on which vessel to replace on May 17, and it will take two or three years to design and build the new vessel, Berg added.

Officials could choose a design with more passenger and vehicle capacity to accommodate growth. The ferry service now carries 1.4 million passengers, 35,000 vehicles and 500,000 pieces of freight a year, Berg said.

“We have to consider that as part of the design,” he added. “We are the lifeline for the islands.”